This invention relates generally to semiconductor technology and more particularly to the method of forming thin film transistors (TFTs) on polycrystalline silicon regions within an amorphous silicon film.
Polycrystalline silicon is formed by crystallizing amorphous silicon films. One method of crystallizing amorphous silicon films is excimer laser annealing (ELA). Conventional ELA processes form polycrystalline films having a random polycrystalline structure. Random, as used here, means that no single crystal orientation is dominant and that polycrystalline structures consist of a mixture of crystallographic orientations in silicon. These crystallographic orientations in silicon are commonly denoted as  less than 111 greater than ,  less than 110 greater than , and  less than 100 greater than , along with their respective corollaries, as is well known in the art. Control of crystallographic orientation is generally desirable because the electrical characteristics of a polycrystalline silicon film depend upon the crystallographic orientation of the film. In addition, the uniformity of the electrical characteristics will improve if the majority of the film has a controllable texture.
ELA, as well as many other annealing methods, has not provided a means to control these microstructural characteristics and achieve a predictable and repeatable preferential crystal orientation and film texture within an annealed film. It would be desirable to have a method of producing TFTs using a polycrystalline silicon film with a more uniform crystallographic orientation. It would also be desirable to be able to produce TFTs using predominantly  less than 100 greater than  polycrystalline silicon.
Accordingly, a method of forming thin film transistor (TFT) structures on a substrate, which has a polycrystalline silicon film with a desired predominant crystal orientation, is provided. The method of forming the TFTs comprises the steps of: providing a substrate, depositing an amorphous silicon film on the substrate, annealing the substrate to produce a polycrystalline film with the desired predominant crystal orientation, preferably a  less than 100 greater than  crystal orientation, polishing the polycrystalline film; forming a gate structure over the polycrystalline film; and doping the polycrystalline film to produce source regions and drain regions.
The substrate can be any material that is compatible with the deposition of amorphous silicon and excimer laser annealing. For display applications, the substrate is preferably a transparent substrate such as quartz, glass or plastic.
To achieve a good quality film that is predominantly  less than 100 greater than crystal orientation, the step of depositing the amorphous film should deposit to a thickness of at least approximately 100 nm.
The step of annealing preferably uses a laterally seeded excimer laser annealing process.
The step of polishing can be accomplished by any means that would not significantly modify the crystal orientation of the film, including by chemical mechanical polishing. The polycrystalline film will preferably be polished to a thickness of less than 100 nm. For some applications a final film thickness of less than 60 nm is desirable.
The method of the present invention, produces a thin film transistor structure comprising a polycrystalline film, which has a predominantly  less than 100 greater than  crystal orientation, overlying a substrate. The final film is preferably less than 100 nm thick. A gate structure overlies the polycrystalline silicon film and source/drain regions are formed by doping the polycrystalline silicon film.